Just posted above, and yeah, that is exactly how I saw it - but it is also the High Sparrow acknowledging Tommen's status and authority. Which no one ever does in this world.

It's not just that the High Sparrow is trying to throw Tommen off by expressing vulnerability. Asking permission to sit also subtly acknowledges Tommen's status. They're on the High Sparrow's turf, but he asks Tommen the king for permission to sit. Would anyone else in Tommen's world do this? I think according to…

Cosima was referring to OncoMice. (Thank you, closed captioning.) Essentially, they're mice genetically modified to make them more susceptibble to cancer, making them better research subjects.